One of apparatuses for displaying an image unit composed of a plurality of dots on a CRT display is described in Japanese Patent Laid-open No. 11390/1982. In the apparatus for displaying an image unit on a CRT display, the image unit is moved on the CRT display in accordance with the subtraction between X value of standard coordinates of the image unit and a vertical standard line, and between Y value of the standard coordinates and a horizontal standard line. In controlling the image unit to be moved on the CRT display, control signals for two adjacent horizontal scanning lines are alternately written into two line buffer memories which are provided in parallel and alternately read from the memories so that image signals read from a character image memory are processed in accordance with the control signals thus read from the line buffer memories, thereby being displayed on the CRT display. The control signals comprise signals of the aforementioned substractions in the X and Y directions so that the image unit is moved smoothly on the CRT display by increasing or decreasing the subtraction signal at an appropriate displaying time.
According to the apparatus for displaying an image unit on a CRT display, however, there is a disadvantage that a memory region is increased because the character image memory is accessed after the control signals for the image unit are once written into the parallel line buffer memories.
There is a further disadvantage that enlarging the size of an image unit is difficult to be performed.
There is a still further disadvantage that, where the number of image units which are designated to be displayed on a CRT display exceeds a predetermined number, an image unit exceeding the predetermined number is not displayed on the CRT display.
There is a yet still further disadvantage that there are provided additional registers into which the so-called "blanking mode" instruction is stored to perform the blanking mode wherein an image unit is moved from the edge of a CRT display to appear thereon or is moved to the edge thereof to disappear therefrom.